The present invention relates to a method for projection exposure applied to photolithography in the field of semiconductor manufacturing. More particularly, it relates to a method in which projection exposure to light of the step-and-scan system is performed while assuring a sufficient depth of focus.
In the field of production of semiconductor devices, 16 MDRAMs in accordance with the design rule of a half-micron (0.5 .mu.m) have already begun to be mass-produced, whereas, as the laboratory work, researches into processing for a sub-half-micron size (up to 0.35 .mu.m) required for the 64 MDRAM of the next generation and for a quarter-micron size (0.25 .mu.m) required for the 256 MDRAM of the next-to-next generation are proceeding. The key technology for the progress in such micro-size processing is photolithography. Past progress in the technology owes much to reduction of the exposing wavelength and high numerical aperture (NA) of a contracting projection lens in a contracting projection exposure device (stepper). The reduction in wavelength and the high NA impose non-meritorious conditions in increasing the depth of focus (DOF) since the DOF is proportional to the exposing wavelength .lambda. and inversely proportional to the square of the numerical aperture NA.
On the other hand, the surface step difference of a semiconductor wafer, as an object to be exposed to light, tends to be increased year by year in keeping with high integration density of the semiconductor integrated circuit. The reason is that, under the current tendency towards three-dimensional device structure, contraction in the three-dimensional direction is not so significant as that in the two-dimensional direction due to necessity in maintaining the performance and reliability of the integrated circuit. If a photoresist material is coated on the surface of a semiconductor wafer having such significant step difference, larger surface step difference or variations in the film thickness are produced on the photoresist layer. On the other hand, the imaging surface cannot be perfectly smooth due to the presence of distortions in the imaging surface, while the substrate surface is slightly tilted from a surface which is perfectly normal to the optical axis of the projection optical system. These factors render difficult the uniform light exposure of the entire wafer in the sole imaging plane.
For overcoming the above problems, several techniques have been proposed in which artifices are used in the methods for exploiting the light exposure device for achieving high contrast and high resolution while maintaining the NA in the projection optical system at a pre-set level and also maintaining the DOF at a practical level.
One of these techniques is the focus latitude enhancement exposure (FLEX) method. This technique resides in carrying out light exposure a plurality of times at the same light exposure position via the same photomask as the imaging surface is shifted for maintaining optical image contrast for an effectively long distance along the optical axis, as disclosed in JP Patent Kokai Publication JP-A-58-17446 (1983). With this technique, at least one of the photomask, wafer and the projection optical system is subjected to wobbling of small amplitudes along the optical axis for shifting the imaging surface. There is also disclosed in e.g., JP Patent Kokai Publication JP-A-63-64037 (1988) a method for shifting the imaging surface by shifting these components stepwise or continuously for each light exposure.
It is up to the device for projection exposure to light to deal with a variety of requirements in addition to the requirement for increasing the DOF. For example, it is required that the device for projection exposure enlarge an area exposed to light by one light exposure operation, that is an exposed area, for handling an increase in chip size brought about by further increases in the integration of semiconductor integrated circuits.
With the conventional step-and-repeat systems for light exposure, in which an exposure area comprising a single chip is exposed at a single time to light and the exposed area is shifted in succession on the substrate, it is necessary to increase the diameter of the projection lens for increasing the exposure area. However, it is extremely difficult, both technically and economically cost, to produce a large-diameter lens free of aberrations.
Thus it has been envisaged to combat the above problem by carrying out light exposure while scanning a beam spot of the illuminating light without exposing a sole exposure area at a time. This system, termed a step-and-scan system, effects light exposure of the wafer with a beam spot from a light source which has passed through a slit and which has a spot area smaller than the area of the photomask. In practice, the photomask and the wafer are moved in the in-plane direction in synchronism with each other while the illuminating optical system and the projection optical system remain stationary.
If this step-and-scan system is applied, the exposure area may be enlarged without the necessity of increasing the diameter of the projection lens.
Meanwhile, the above-described step-and-scan system, while coping with the increases chip size, cannot sufficiently cope with the requirement of increasing the depth of focus (DOF).